basically SN1 mechanism has a carbo cation intermediate.. so if a carbo cation is formed and is stabilized by any factors such as resonance inductive effect hyper conjugation.. SN1 mechanism will take place...

p.s it is just one of the factors.. but an imp one.. there are more like it takes place in polar protic solvents(polar solvents which contain h+)

In both the cases only alkyl groups will be attacked not the phenyl group.
In first case, due to minimal steric hindrance SN2 mechanism will be followed.
In second case, due to formation of 3° carbocation SN1 mechanism will be followed.